1. Field of the Invention
The present invention relates to a fuel delivery system for an internal combustion engine, and more particularly to a fuel enrichment device having multiple chambers.
2. Description of Related Art
A hand-held power device such as a chainsaw, hedge trimmer, line trimmer or edger is often powered by small internal combustion engine outfitted with a diaphragm carburetors. Generally, a diaphragm carburetor has an air passage with a venturi, a diaphragm pump, a needle valve and a metering chamber containing a spring-biased diaphragm. The outlet of the air passage leads to an intake tract or crankcase of the engine. A throttle valve of the butterfly type is typically mounted in the air passage to control the amount of fuel and air entering the intake tract or crankcase.
Fuel is drawn into the carburetor by the diaphragm pump, which is connected to the metering chamber through the needle valve. The metering chamber, in turn, is connected to the air passage through supply passages fitted with one-way valves. The supply passages open to the air passage through a plurality of outlet ports. The opening and closing of the needle valve and, thus, the flow of fuel into the metering chamber is controlled by a spring-biased diaphragm, which is mounted inside the metering chamber.
During normal operation of the engine, pulses of pressure from the engine cause the diaphragm pump to pump fuel from a storage tank up to the needle valve. Subatmospheric air pulses passing through the venturi create a negative pressure in the metering chamber, causing a displacement of the metering chamber diaphragm. The displacement of the diaphragm opens the needle valve and permits fuel to enter the metering chamber. The fuel exits the metering chamber through the outlet ports and enters the air passage where it is atomized. Eventually, the flow of fuel into the metering chamber increases the pressure in the metering chamber, causing the diaphragm to close the needle valve and stop the flow of fuel. As the fuel empties from the metering chamber, the pressure in the metering chamber drops until the diaphragm is again displaced and the needle valve opens. In this manner, the diaphragm in the metering chamber continually opens and closes the needle valve, thereby introducing metered amounts of fuel into the air passage.
Since the delivery of fuel in a diaphragm carburetor is not dependent upon gravity, the operation of a diaphragm carburetor is not affected by its orientation. Accordingly, diaphragm carburetors are ideally suited for use in power devices such as chainsaws that may be held by an operator in a variety of positions. Engines utilizing diaphragm carburetors, however, tend to be difficult to start after a period of non-use because of an initial absence of fuel in the metering chamber and the diaphragm pump. Air choke mechanisms are utilized to remedy this situation. However, most air choke mechanisms are unable to quickly and efficiently establish a proper air to fuel ratio and can flood the engine by introducing excess fuel into the engine.
Air choke mechanisms are usually comprised of slide valves or butterfly valves. Typically, a butterfly valve will be rotatably mounted inside the air passage near the inlet. The butterfly valve often has a small orifice passing therethrough. Usually, the butterfly valve can be rotated between three different positions: an open position, a half-choke position and a full choke position. When the butterfly valve is in the open position, the inlet to the air passage is substantially open. In the half-choke position, the butterfly valve is partially closed and, thus, partially blocks the inlet to the air passage. In the full-choke position, the butterfly valve is closed and blocks the inlet to the air passage except for the small orifice. When the engine is cranked during starting, by a pull rope or otherwise, air is drawn out of the air passage and into the engine. If the choke mechanism is in a full-choke position or a half-choke position, the withdrawal of air creates a negative pressure condition in the air passage. Of course, the amount of pressure reduction is greater in the full-choke position than in the half-choke position. The negative pressure in the air passage creates a negative pressure in the metering chamber which displaces the diaphragm and allows fuel to enter the metering chamber and thence the air passage, where it mixes with air to create an air/fuel mixture.
During the initial cranking cycle, the choke mechanism is placed in a full-choke position to create a maximum vacuum in the air passage. In addition, the throttle valve is fully opened to permit the maximum vacuum to be applied to the outlet ports so as to create a maximum fuel draw. The opening of the throttle valve also permits a maximum amount of the air/fuel mixture to reach the intake tract or crankcase of the engine. In the full-choke position, however, the air/fuel mixture is very fuel-rich since only a small quantity of air can enter the air passage through the choke mechanism. As the engine begins to fire, more air is required to provide an adequate air/fuel ratio to keep the engine running. Accordingly, the choke mechanism must be moved to the half-choke position as soon as the first internal explosion, or “pop” occurs in the engine. If the choke mechanism is left in the full-choke position for too many cranking cycles after the “pop” occurs, the engine will become flooded with fuel and will not start. The engine will have to be allowed to rest long enough to permit the excess fuel in the crankcase and/or the combustion chamber to evaporate and a proper fuel-air mixture to be restored.
In the half-choke position, the choke mechanism increases the air content in the air/fuel mixture, but still provides a rich-running condition required by the engine during warm-up. After the engine has been running for a few seconds, the choke mechanism must be moved from the half-choke position to the open position to provide a correct air/fuel ratio.
In addition to an air choke, the engine's air/fuel delivery system can include a manually operated fuel enrichment device or primer that forces extra fuel into the carburetor's air passage during starting and during warm-up. The extra fuel results in an enriched air/fuel mixture.
Operation of the choke and primer systems discussed above require a skilled operator. An unskilled operator improperly applying the choke or primer could accidentally flood the engine. A priming system for enriching the air/fuel mixture automatically without requiring any positive action by the operator would be desirable.